The present invention relates generally to processing audio signals, and more particularly to a method and apparatus for processing an audio signal to compensate for the frequency response of loudspeakers.
Devices that reproduce acoustic signals such as, for example, television sets, radio receivers, and stereo systems, include a circuit for processing the audio signals. Typically, such circuits are designed to compensate for the frequency response of the loudspeakers to improve acoustic reproduction, and to prevent overdriving the device or the system.
The most critical element in a unit for acoustic reproduction is the loudspeaker. The acoustic pressure of a loudspeaker drops about 40 db per decade below a structurally determined limit frequency. This corresponds to the transfer function of a second-order filter. On the other hand, bass reflex and transmission line loudspeakers have transfer functions which correspond to a higher order filter. Their lower limit frequency typically lies between about 50 Hz and 200 Hz. The lower the limit frequency of a loudspeaker, the more expensive it is to produce. Economical units such as, for example, television or portable radio receivers consequently are equipped with simple loudspeakers that have a relatively high lower limit frequency. To improve acoustic reproduction in the lower frequency range, the limit frequency of such units is shifted downward by preamplifying the low frequencies. However, this can cause the final amplifier and the loudspeakers to be overdriven. To prevent overdriving and possibly even destruction of the final amplifier or the loudspeaker, the output signal of the bass amplifier is fed back in such a way that the amplification of the lower frequencies is reduced if the output signal is strong. One example of such an approach is disclosed in U.S. Pat. No. 5,305,388.
Another conventional approach is disclosed in U.S. Pat. No. 5,359,665. This patent describes a circuit in which the audio signal is conducted directly, via a first path, to the first input of an adder. The audio signal is simultaneously conducted, via a second path, to the second input of the adder. This second path includes a low-pass filter and an amplifier with variable amplification. The output of the amplifier is fed back, through a signal level detector, to its control input. This circuit arrangement is described in the '665 patent to have the benefit of reducing overdrive of the final amplifier.
From research results of psychoacoustics, it is known that a person can still unambiguously determine the fundamental level of a tone even when only harmonics of the fundamental frequency and not the fundamental frequency itself is present in the spectrum. This psychoacoustic effect is utilized in that the harmonic of the fundamental frequency is generated and is conducted to a loudspeaker whose limit frequency lies above this fundamental frequency. A listener consequently believes that he or she hears this low fundamental frequency even though the loudspeaker does not radiate it at all.
U.S. Pat. No. 5,668,885 describes a circuit that “entices” from a loudspeaker, having a relatively high lower limit frequency, frequencies which are lower than its limit frequency. This is done by generating harmonics of the lower frequencies. The audio signal is conducted, via a first path, to the first input of an adder. In a second path, the audio signal passes through a low-pass filter, is rectified, passes through a second low-pass filter, is amplified, and finally is conducted to the second input of the adder.
U.S. Pat. No. 4,150,253 describes a method and a circuit in which an audio signal is divided into two signal paths. In the first path, the audio signal traverses a high-pass filter, so as to shift the phase based on the frequency. Those signals at the output of the high-pass filter which are at levels that exceed a given value are conducted to the input of a generator which generates the harmonics of the fundamental frequency. The level of the signals at the output of the generator is attenuated to a value below the level of the original audio signal. This attenuated signal and the original audio signal are then added together.
U.S. Pat. No. 4,700,390 describes a synthesizer in which harmonics are generated both for the lower and higher frequencies, and are added to the original audio signal. The alleged benefit set out in the '390 patent is that this achieves better reproduction both in the low and high frequency ranges.
U.S. Pat. No. 5,771,296 describes a circuit in which the audio signal is conducted, via a first path, directly into an adder, while, via a second path, the harmonics of the lower frequencies are generated and are added in the adder to the original signal. This provides the perception to the listener that the loudspeaker radiates lower frequencies than it actually does.
U.S. Pat. No. 4,739,514 describes a circuit to improve the acoustic reproduction of low frequencies. With this circuit, the audio signal is conducted, via a first path, to the first input of an adder, while, via a second path consisting of an amplifier with variable amplification in series with a bandpass filter, it is conducted to the second input of the adder. A signal level detector, whose input receives the audio signal, controls the amplification of the amplifier.
The above and other conventional methods and circuits have the disadvantage that, due to the feedback technique employed, they react relatively slowly to rising amplitudes and, despite the feedback, they tend to overdrive the device or the system.
What is needed, therefore, is an audio processing technique that compensates for the frequency response of a loudspeaker without overdriving the circuit, its components or the entire reproduction system, particularly in the range of low frequencies.